Polymer “Clicking” by CuAAC Reactions

The CuAAC: Principles, Homogeneous and Heterogeneous Catalysts, and Novel Developments and Applications

The copper-catalyzed azide/alkyne cycloaddition reaction (CuAAC) has emerged as the most useful "click" chemistry. Polymer science has profited enormously from CuAAC by its simplicity, ease, scope, applicability and efficiency. Basic principles of the CuAAC are reviewed with a focus on homogeneous and heterogeneous catalysts, ligands, anchimeric assistance, and basic chemical principles. Recent developments of ligand design and acceleration are discussed.

Metal-Free Fast Azidation by Using Tetrabutylammonium Azide: Effective Synthesis of Alkyl Azides and Well-Defined Azido-End Polymethacrylates

An effective method to synthesize azido-end polymethacrylates from tetrabutylammonium azide (BNN₃ ) in a nonpolar solvent (toluene) was developed. Several low-mass alkyl halides were reacted with BNN₃ in toluene as model reactions and the rate constants of these reactions were determined, to confirm fast BNN₃ azidation for tertiary and secondary halides. The end-group transformation of halide-end polymethacrylates was effective and nearly quantitative. Notably, the combination of organocatalyzed living (or reversible deactivation) radical polymerization and BNN₃ azidation enabled the metal-free synthesis of azido-end polymethacrylates, including single-azido-end and multi-azido-end functional homopolymers and block copolymers. The rapid and quantitative reaction without the requirement for a large excess of BNN₃ , metal-free and polar-solvent-free nature, and broad polymer scope are attractive features of this azidation.

Synthesis, characterization and catalytic activity of novel ruthenium complexes bearing NNN click based ligands

Novel air stable ruthenium(ii) complexes bearing tridentate ligands bis((1-benzyl-1H-1,2,3-triazol-4-yl)methyl)amine (L1), 1-(1-benzyl-1H-1,2,3-triazol-4-yl)-N-(pyridin-2-ylmethyl)methanamine (L2) or 2-(4-phenyl-1H-1,2,3-triazol-1-yl)-N-(pyridin-2-ylmethyl)ethan-1-amine (L3) were synthesised. The nitrogen based ligands were easily prepared by virtue of click chemistry using cheap and commercially available reagents. The ruthenium complexes were obtained by heating the Ru(PPh3)3Cl2 precursor and the tridentate NNN ligand in toluene under reflux for 2 hours, achieving yields of 82-87%. These complexes were fully characterized by means of NMR, FT-IR and high resolution ESI spectroscopy. The crystal structure of one of the complexes was determined. These complexes showed excellent activity and selectivity in the hydrogenation of ketones and aldehydes. DFT calculations show that complex 3 may react through an outer-sphere catalytic cycle rather than via an inner-sphere mechanism.

Sequence information transfer using covalent template-directed synthesis

Kinetically inert ester bonds were used to attach monomers to a template, dictating the sequence of the polymer product.

Template-directed synthesis is the biological method for the assembly of oligomers of defined sequence, providing the molecular basis for replication and the process of evolution. To apply analogous processes to synthetic oligomeric molecules, methods are required for the transfer of sequence information from a template to a daughter strand. We show that covalent template-directed synthesis is a promising approach for the molecular replication of sequence information in synthetic oligomers. Two monomer building blocks were synthesized: a phenol monomer and a benzoic acid monomer, each bearing an alkyne and an azide for oligomerization via copper catalyzed azide alkyne cycloaddition (CuAAC) reactions. Stepwise synthesis was used to prepare oligomers, where information was encoded as the sequence of phenol (P) and benzoic acid (A) units. Ester base-pairing was used to attach monomers to a mixed sequence template, and CuAAC was used to zip up the backbone. Hydrolysis of the ester base-pairs gave back the starting template and the sequence complementary copy. When the AAP trimer was used as the template, the complementary sequence PPA was obtained as the major product, with a small amount of scrambling resulting in PAP as a side-product. This covalent base-pairing strategy represents a general approach that can be implemented in different formats for the replication of sequence information in synthetic oligomers.

Synthesis, characterization and chemical degradation of poly(ester-triazole)s derived from d-galactose

α-Azide-ω-alkynyl ester monomers were designed and synthesized in order to obtain hydrolytically degradable polymers. The monomers were prepared from d-galactose, as a renewable resource. Environmentally benign azido–alkyne cycloaddition polymerizations were conducted to afford poly(ester-triazole)s, with complete atom economy. Although polymer formation prevailed under optimized polymerization conditions, variable proportions of cyclic oligomer byproducts were detected. The Cu-catalyzed click polymerization led regioselectively to 1,4-disubstituted triazole linkages, while the thermal, metal-free polymerization produced a random distribution of 1,4- and 1,5-disubstituted triazoles in the polymer backbone. The poly(ester-triazole)s exhibited high molecular weights (M_w in the range 35–85 kDa). They were soluble in organic solvents but highly insoluble in water, thus removal of the Cu(i) catalyst was simplified. The polymers were stable up to 300 °C, and had T_g values in the range 90–100 °C. The materials were hydrolysed under either basic or strong acid conditions, and the degradation products have been characterized.

Carbohydrate-derived poly(ester-triazoles), soluble in organic solvents and degradable in aqueous media, have been synthesized by CuAAC or thermal polymerization.

Functional polyamides withgem-diazido units: synthesis and diversification

Polyamide structures bearing geminal diazide units were constructed with diazidated malonates and diamines.

Synthesis of polymer-linked copper(i) bis(N-heterocyclic carbene) complexes of linear and chain extended architecture

Novel PS-based mechanophores of linear and chain-extended architecture are synthesized obtaining bis(NHC) complexes with more than one Cu(i) center per chain and molecular weights of up to 50 000 g mol⁻¹.

Cap control: cyclic versus linear oligomerisation in covalent template-directed synthesis

Covalent template-directed synthesis was used to oligomerise monomer building blocks in a controlled manner to give exclusively the linear trimer. Competing reaction pathways were blocked by addition of a large excess of a monomeric capping agent. At a concentration of 1 mM, the cap selectively prevented further reaction of the product chain ends to give polymeric and macrocyclic products, but did not interfere with the templating process.

The right concentration of capping agent is required to control the product distribution in covalent template-directed synthesis of linear oligomers using CuAAC.

Copper Ligand and Anion Effects: Controlling the Kinetics of the Photoinitiated Copper(I) Catalyzed Azide-Alkyne Cycloaddition Polymerization

The kinetics of photoinduced copper(I) catalyzed azide-alkyne cycloaddition (CuAAC) polymerizations were assessed as a function of copper(II) amine-based ligands. Copper(II) bromide ligated with 1,1,4,7,10,10-hexamethylenetetramine (HMTETA) exhibited the fastest kinetics in both Norrish type(I) and type(II) photoinitiating systems. A characteristic induction period is observed with these polymerizations and is manipulated by adding an external tertiary amine in Norrish Type(II) photoinitating systems or by changing the anion of the copper(II) salt. Halides, specifically bromide and chloride, exhibit the fastest kinetics with the smallest induction period in comparison with organic anions, such as bistriflimide and triflate. The temporal control of the photo-CuAAC polymerization is affected by pre-ligation of the copper catalyst, by the presence of certain anions such as acetate, and by specific ligands such as tetramethylethylenediamine (TMEDA).

Reactive nano-patterns in triple structured bio-inspired honeycomb films as a clickable platform

Towards unprecedented triple structured bio-inspired honeycomb film by selfassembly of a functional block copolymer during breath figure templating as a nano-patterned clickable platform.

Direct access to biocompatible nitroxide containing polymers

Ring-opening copolymerization of a nitroxide containing cyclic carbonate and d/l-lactide was used to directly access well-defined biocompatible polymers.

CuAAC-Based Click Chemistry in Self-Healing Polymers

Click chemistry has emerged as a significant tool for materials science, organic chemistry, and bioscience. Based on the initial concept of Barry Sharpless in 2001, the copper(I)-catalyzed azide/alkyne cycloaddition (CuAAC) reaction has triggered a plethora of chemical concepts for linking molecules and building blocks under ambient conditions, forming the basis for applications in autonomous cross-linking materials. Self-healing systems on the other hand are often based on mild cross-linking chemistries that are able to react either autonomously or upon an external trigger. In the ideal case, self-healing takes place efficiently at low temperatures, independent of the substrate(s) used, by forming strong and stable networks, binding to the newly generated (cracked) interfaces to restore the original material properties. The use of the CuAAC in self-healing systems, most of all the careful design of copper-based catalysts linked to additives as well as the chemical diversity of substrates, has led to an enormous potential of applications of this singular reaction. The implementation of click-based strategies in self-healing systems therefore is highly attractive, as here chemical (and physical) concepts of molecular reactivity, molecular design, and even metal catalysis are connected to aspects of materials science. In this Account, we will show how CuAAC reactions of multivalent components can be used as a tool for self-healing materials, achieving cross-linking at low temperatures (exploiting concepts of autocatalysis or internal chelation within the bulk CuAAC and systematic optimization of the efficiency of the used Cu(I) catalysts). Encapsulation strategies to separate the click components by micro- and nanoencapsulation are required in this context. Consequently, the examples reported here describe chemical concepts to realize more efficient and faster click reactions in self-healing polymeric materials. Thus, enhanced chain diffusion in (hyper)branched polymers, autocatalysis, or internal chelation concepts enable efficient click cross-linking already at 5 °C with a simultaneously reduced amount of Cu(I) catalyst and increased reaction rates, culminating in the first reported self-healing system based on click cycloaddition reactions. Via tailor-made nanocarbon/Cu(I) catalysts we can further improve the click cross-linking reaction in view of efficiency and kinetics, leading to the generation of self-healing graphene-based epoxy nanocomposites. Additionally, we have designed special CuAAC click methods for chemical reporting and visualization systems based on the detection of ruptured capsules via a fluorogenic click reaction, which can be combined with CuAAC cross-linking reactions to obtain simultaneous stress detection and self-healing within polymeric materials. In a similar concept, we have prepared polymeric Cu(I)-biscarbene complexes to detect (mechanical) stress within self-healing polymeric materials via a triggered fluorogenic reaction, thus using a destructive force for a constructive chemical response.

Click and Click-Inspired Chemistry for the Design of Sequence-Controlled Polymers

During the previous decade, many popular chemical reactions used in the area of "click" chemistry and similarly efficient "click-inspired" reactions have been applied for the design of sequence-defined and, more generally, sequence-controlled structures. This combination of topics has already made quite a significant impact on scientific research to date and has enabled the synthesis of highly functionalized and complex oligomeric and polymeric structures, which offer the prospect of many exciting further developments and applications in the near future. This minireview highlights the fruitful combination of these two topics for the preparation of sequence-controlled oligomeric and macromolecular structures and showcases the vast number of publications in this field within a relatively short span of time. It is divided into three sections according to the click-(inspired) reaction that has been applied: copper-catalyzed azide-alkyne cycloaddition, thiol-X, and related thiolactone-based reactions, and finally Diels-Alder-chemistry-based routes are outlined, respectively.

SuFEx-Based Polysulfonate Formation from Ethenesulfonyl Fluoride-Amine Adducts

The SuFEx-based polycondensation between bisalkylsulfonyl fluorides (AA monomers) and bisphenol bis(t-butyldimethylsilyl) ethers (BB monomers) using [Ph₃ P=N-PPh₃ ]⁺ [HF₂ ]^- as the catalyst is described. The AA monomers were prepared via the highly reliable Michael addition of ethenesulfonyl fluoride and amines/anilines while the BB monomers were obtained from silylation of bisphenols by t-butyldimethylsilyl chloride. With these reactions, a remarkable diversity of monomeric building blocks was achieved by exploiting readily available amines, anilines, and bisphenols as starting materials. The SuFEx-based polysulfonate formation reaction exhibited excellent efficiency and functional group tolerance, producing polysulfonates with a variety of side chain functionalities in >99 % conversion within 10 min to 1 h. When bearing an orthogonal group on the side chain, the polysulfonates can be further functionalized via click-chemistry-based post-polymerization modification.

Design of a thermally controlled sequence of triazolinedione-based click and transclick reactions

The reaction of triazolinediones (TADs) and indoles is of particular interest for polymer chemistry applications, as it is a very fast and irreversible additive-free process at room temperature, but can be turned into a dynamic covalent bond forming process at elevated temperatures, giving a reliable bond exchange or 'transclick' reaction. In this paper, we report an in-depth study aimed at controlling the TAD-indole reversible click reactions through rational design of modified indole reaction partners. This has resulted in the identification of a novel class of easily accessible indole derivatives that give dynamic TAD-adduct formation at significantly lower temperatures. We further demonstrate that these new substrates can be used to design a directed cascade of click reactions of a functionalized TAD moiety from an initial indole reaction partner to a second indole, and finally to an irreversible reaction partner. This controlled sequence of click and transclick reactions of a single TAD reagent between three different substrates has been demonstrated both on small molecule and macromolecular level, and the factors that control the reversibility profiles have been rationalized and guided by mechanistic considerations supported by theoretical calculations.

Biocompatible Soft Nanoparticles with Multiple Morphologies Obtained from Nanoprecipitation of Amphiphilic Graft Copolymers in a Backbone-Selective Solvent

Stealth nanocarriers are a promising technology for the treatment of diseases. However, the preparation and characterization of well-defined soft nanoparticulate systems remain challenging. Here we describe a platform of amphiphilic graft copolymers leading to nanoparticles with multiple morphologies and the role of the hydrophilic backbone in their interaction with a model protein. The amphiphilic graft copolymers platform was composed of hydrophilic backbone poly(2-methyl-2-oxazoline-co-2-pentyl-2-oxazoline) (P(MeOx-co-PentOx)), prepared via cationic ring-opening polymerization. Hydrophobic poly(d,l-lactide) (PLA) chains were grafted on the backbone via Huisgen 1,3-dipolar cycloaddition. The "click" copper-catalyzed cycloaddition reactions of azides with alkynes (CuAAC) were successfully carried out, and a series of amphiphilic copolymers were prepared containing a backbone with a number-average molecular weight of 14.2 × 10³ g mol^-1 and different hydrophobic PLA grafts with various molecular weights (2.8 × 10³-12.4 × 10³ g mol^-1). These original architectures of copolymers, when nanoprecipitated in water, the backbone-selective solvent, allowed us to obtain various structures of nanoparticles with a hydrodynamic diameter in the range of 65-99 nm. More interestingly, a plurality of morphologies going from unilamellar, multilamellar, and large compound vesicles to core-shell nanoparticles and depending on the PLA molecular weights were evidenced by combining cryo-transmission electron microscopy (cryo-TEM) and small-angle neutron scattering (SANS) studies. A first evaluation of their stealthiness by studying the stability and the interaction of these nano-objects with a model protein revealed the role played by the P(MeOx-co-PentOx) in these interactions, demonstrating the utility of this amphiphilic graft copolymers platform with well-defined architectures for the design of nanocarriers in drug delivery applications.

A straightforward and sustainable synthesis of 1,4-disubstituted 1,2,3-triazoles via visible-light-promoted copper-catalyzed azide–alkyne cycloaddition (CuAAC)

A simple and environmentally friendly synthesis of triazoles through the effective reduction of copper(ii) assisted by organic dyes and promoted by visible light was developed.

Exposure to air boosts CuAAC reactions catalyzed by PEG-stabilized Cu nanoparticles

The catalytic activity of Cu(0) NPs is boosted upon aerobic oxidation, forming Cu₂O NPs, and further improved on an SBA-15 support.

Simultaneous doping and crosslinking of polythiophene films

We present a click chemistry approach for the synthesis of conjugated redox polymers based on highly regioregular polythiophenes with tunable amounts of pendant redox-active triphenylamine (TPA) groups. Solution-deposited films can be simultaneously doped and crosslinked by electrochemical or chemical oxidation.